This invention relates to an improvement in a method for producing a high density metal article from a metal powder involving cold pressing and sintering the powder, the improvement being hot isostatic pressing of the cold pressed and sintered article.
The usual technique used to produce metal parts is via powder metallurgical processing. Instead of melting solid metal, pouring it into molds and removing the part from the mold, with powder metallurgy, the starting material is a metal powder. The metal powder is formed to a desired shape and then sintered or heated to convert it to solid metal. Basically, this is a two-step operation involving cold isostatic pressing in a high pressure vessel and sintering in a furnace. Since the part contracts or shrinks when made by this method, a larger starting shape must be used to produce the desired finished part. For instance, the density of the metal in powder form is normally 25% to 40% of theoretical density (very porous), the density after pressing (green state) is normally 60% to 70% of theoretical density and the sintered part is normally 93% to 97% of theoretical density. Thus, the shrinkage effect can be seen -- as the density increases, the porosity decreases and thus the size and volume decreases since the weight remains the same. The final size is divided by a shrink factor which provides the mold starting size the powder will contact. Molds are typically rubber bags which are placed inside of steel containers that maintain the shape of the molds. Metal powder is placed inside the mold which is the desired shape (round, square, tubular, etc.) and this mold is sealed either by liquid rubber or by mechanical means such as clamps. Thus, the powder is totally enclosed by rubber. This is required since the entire assembly will be placed in a high-pressure vessel which uses an oil or water medium as a pressing agent and this step is referred to as cold isostatic pressing. The part is placed in the press, the press is sealed, water or oil is pumped into the vessel at high pressure (20 ksi to 45 ksi), the part is now in the as-pressed or "green" state in which the part can be handled but will chip or break if dropped, hammered, etc. The part is then placed in a furnace, heated or sintered below its melting point resulting in a powder to solid transformation -- coalescing of powder particles, and formation of metallic grains and grain boundaries. Thus a sintered or solid metal part is produced.
Powder metallurgy is used for two main reasons:
1. It reduces the starting material weight since parts can be formed to near net shape. This also reduces machining time if the part is to be machined.
2. It allows a lower usage of energy to produce solid metallic parts-sintering temperatures are much lower than required melting temperatures if the parts are to be cast.
One disadvantage of powder metallurgically produced parts is that full densification cannot be achieved unless sintered for an extremely long duration of time which results in undesirable grain growth and decreased properties.
Therefore, a process for producing essentially fully densified metal parts without excessively long sintering times would be an advancement in the art.